President-elect Donald Trump has already begun to back off some of his promises: Maybe not all of Obamacare has to go. Maybe parts of his wall will actually be a fence. Maybe it’s okay to have some lobbyists running the government after all.

But I fear he won’t shrink from the actions he has promised on climate change: withdrawing the United States from the Paris accord, ending President Obama’s Clean Power Plan and okaying every new fossil-fuel plan from the Keystone XL pipeline on down. He won’t back down because those are hard-to-hedge choices and because he’s surrounded by climate-change deniers and fossil-fuel insiders who will try to ensure that he keeps his word.

So let’s be entirely clear about what those actions would represent: the biggest, most against-the-odds and most irrevocable bet any president has ever made about anything.

It’s the biggest because of the stakes. This year has been the hottest year recorded in modern history, smashing the record set in 2015, which smashed the record set in 2014. The extra heat has begun to steadily raise sea levels, to the point where some coastal U.S. cities already flood at high tide even in calm weather. Global sea ice levels are at record lows, and the oceans are 30 percent more acidic. And that’s just so far. Virtually every scientific forecast says that without swift action in the next few years to cut carbon emissions, this crisis will grow to be catastrophic, with implications for everything from agriculture to national security that dwarf our other problems.

It’s the most against-the-odds bet because at this point there’s so little scientific dispute about climate change. Researchers have spent three decades narrowing the error bars and establishing an ever-clearer picture of the future. There’s always the chance that scientists have overlooked something, but it’s by now so narrow a chance it hardly deserves that description.